make dependency on parts of a file

Suppose I have a file A and another file B depending on only parts of A.
That is, if (the time stamp of) A changes, it is quite likely that B
(derived from A) will not.
Suppose it's cheap to build B from A (or to check whether B actually changed).
Then suppose there's C, which depends on B but is expensive to build.
Is there a sane way to rebuild C only if the relevant parts of A changed?
If you prefer actual examples, A is my network configuration file,
B is the list of Access Points and C is the RADIUS client configuration.
The thing I'm trying to avoid is restarting the RADIUS server each time
something changed in the network config.